The invention relates to a drive for presses, in particular cutting and/or forming presses, having pressurized drive cylinders which drive a press ram in interaction with articulated elements. If two or more pressure points are used for initiating displacement or force at the ram, the articulated elements are synchronized in a controlled manner. A plurality of rams may also be driven by the use further of [sic] coupling articulations.
Continuous-path-control and adaptive-control presses differ, inter alia, according to the type of press drive. Mechanically driven presses work with continuous-path control, i.e. not until the kinematic drive means, e.g. an eccentric, approaches bottom dead center does the desired forming or rated force greatly increase. In contrast, in the adaptive-control, hydraulically driven press, the maximum forming force is available in every ram position. Set against this advantage of the hydraulically driven press are a number of considerable disadvantages, such as, for example, low number of strokes and high drive power to be installed with poor overall efficiency.
A detailed account of the advantages and disadvantages of a hydraulic press compared with a mechanical press may be gathered from the article xe2x80x9cEine Stxc3xcckkosten-Analyse erleichtert die Wahlxe2x80x9d [A unit cost analysis facilitates choice], published in Industrieanzeiger 16/97, pages 30 and 31.
A disadvantage of the mechanically driven press is the complicated and expensive drive, consisting of, for example, motor, clutch/brake combination, flywheel with drive, gear chain with gears, eccentric wheels, connecting rod, shafts and bearings. The attempt to combine the advantages of a hydraulically driven press with those of a mechanical press can be seen from EP 0 616 882. The object of the latter was to achieve an essentially constant forming speed during the entire forming operation. The contents of this publication are essentially the hydraulic activation of the working cylinder and the open-loop and closed-loop control blocks required for this; the actual drive kinematics are not a subject matter of the patent rights. The kinematics shown in diagrammatic sketches correspond to the known toggle-lever or toggle-joint drive known from die-casting or injection-molding machines. This toggle-joint drive, in particular when approaching the extended position of the articulations, requires very large displacements of the drive cylinder with correspondingly large oil quantities.
The object of the invention is to propose a drive system for a press, which drive system, with a low drive power, achieves a higher number of strokes than a hydraulically driven press and is more cost effective than a mechanical press due to a reduction in the drive elements without dispensing with the controlled synchronization, present in mechanical presses, of the force- or displacement-initiating components.
The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate presently preferred embodiments of the invention and, together with the general description given above and the detailed description given below, serve to explain features of the invention.
The invention is based on the idea of designing the drive system in such a way that the complete ram stroke, i.e. downward and upward stroke, can be performed with a single cylinder stroke in one direction without reversing connection. In this case, any desired ram stroke from the minimum to the maximum displacement is to be capable of being performed with this drive concept. Minimum strokes are realized for pure cutting operations, whereas maximum strokes, for example, are required when using transfer systems with corresponding degrees of freedom. In particular in the case of a ram having a large area, a plurality of drive points are necessary in order to avoid tilting of the ram. In this case, mechanical controlled synchronization is provided between the drive levers.
The motion characteristic of the proposed lever systems is characterized by a continually increasing transmission ratio in the direction of bottom dead center and thus by a correspondingly increasing press force.
In contrast to the conventional hydraulic press, a drive cylinder of substantially smaller diameter may therefore be selected; the number of strokes is markedly increased despite the lower drive power to be installed. The use of a so-called synchronous cylinder is especially advantageous for producing identical stroke or force ratios with the simplest closed-loop control. Since in each case the same diametral and thus area ratios are present in this cylinder in both stroke directions, there is also a very favorable control response.
Due to the smaller drive cylinder, the oil quantity required for operation is also reduced, as a result of which the undesirable compressibility of the pressure medium decreases. However, on account of the lever mechanisms connected in between, this compressibility is only of secondary importance, which has a favorable effect in particular for the reduction of the cutting impact occurring during cutting operations.
The motion and speed profile known from the mechanical presses also has an advantageous effect. In the cutting and forming region, the force increases on the one hand, but the speed is reduced. This speed, which continually decreases toward bottom dead center, is necessary in particular during the material flow during the forming operation in order to obtain good workpieces.
This advantageous kinematic sequence of motion is achieved by a considerably small number of components compared with a mechanical press. The press becomes more cost-effective and thus has corresponding efficiency with optimum sequences of motion from the forming point of view.
As already mentioned, any desired size of stroke can be performed with the proposed drive within the limits of the predetermined geometry. To this end, the drive cylinder may be provided with a displacement measuring system, which can be extended in combination with a displacement measuring system at the ram to form a position control loop. At the same time, it is also ensured in this application that the cylinder in each case travels only in one direction for a complete ram stroke, i.e. downward and upward movements. The ram stroke required in each case is then realized by the size of the associated stroke of the drive cylinder. Thus different ram strokes, e.g. for a drawing operation with subsequent sizing impact, are also possible in a forming process.
A controlled drive cylinder, by imposing a corresponding speed profile, permits any desired speed behavior of the ram. In addition to the forming process, this also offers considerable advantages during the use of automation equipment by optimizing the degrees of freedom.
A further embodiment according to the invention is shown by way of example by the drive of a plurality of rams through the use of additional coupling articulations. This arrangement is desirable, for example, in transfer presses which require a superimposed cutting ram for cutting die plates.
Further details and advantages of the invention follow from the description below of exemplary embodiments.